<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
  <style>
    html, body{
      padding: 0;
      margin: 0;
    }
    .wrap{
      height: 100vh;
      display: flex;
      flex-direction: column;
    }
    .box1{
      height: 300px;
      background-color: aquamarine;
    }
    .box2{
      background-color: brown;
      flex: 1;
    }
    .inner{
      height: 100%;
    }
  </style>
</head>
<body>
  <div class="wrap">
    <div class="box1"></div>

    <div class="box2">
      <div class="inner">hello</div>
    </div>
  </div>
</body>
</html>